1. Field of the Invention
The present invention relates to bearing structures, and in particular, thrust washers.
2. Description of the Related Art
Thrust washers and related force transferring members are used in a variety of situations to transmit mechanical force. FIG. 1 illustrates a prior art final drive of an agricultural vehicle using such a thrust washer. The present invention concerns the region of the thrust washer, not the final drive as a whole, and the final drive as a whole therefore will not be described in detail here. Details may be found in U.S. Pat. No. 4,693,699 (Gregerson).
In the art structure, thrust must be transferred between a rotating part 10 and a non-rotating part 12. The prior art technique used a thrust washer 14 having a rectangular cross-section to do this. The thrust washer fits in a groove 16 formed in the rotating part 10 and abuts against a shoulder 18 formed in the non-rotating part 12. The thrust washer 14 itself is formed in sections, e.g., two or three sections, so that it can be positioned in the groove without stretching or bending.
While simple, this structure suffers from problems of stress. Specifically, the sharp right angles in the thrust washer 10 and shoulder 18 create high stresses at their intersections.
A similar situation was encountered by the inventors in U.S. Pat. No. 4,288,172 (Livesay et al.). They concluded that it would be preferable to round the shoulder and use a rounded, rather than rectangular cross-section, thrust washer. Livesay et al. teach two possible structures with rounded corners: their preferred structure is a solid metal washer with a top surface which includes 180.degree. arc. Their alternative structure has a 90.degree. arc. They found that the 90.degree. arc structure can withstand a much higher degree of stress than the 180.degree. arc structure. However, Livesay et al. nevertheless teach that the 180.degree. structure is preferable because a 90.degree. structure would be extremely difficult to manufacture, and because it has the distinct disadvantage that it can be inadvertently inserted backwards.
For reasons of manufacturability and durability, it would be preferable to make the thrust washer out of sintered powdered metal. However, a 180.degree. curved surface made out of sintered metal would be extremely difficult to manufacture. FIG. 3 illustrates why: To make such a structure, a die 20 would be provided with a groove having a curved side 22. Similarly, the punch 24 used to compress the powdered metal 26 includes a curved part 28. The problem is that the curved part 28 must curve down to a very thin end. In a relatively short time, this thin end would wear and the punch would no longer produce a satisfactory shape. This would require frequent replacement of the punch, so that the manufacturing process would be very expensive.